lighthouses of the universe - gbv - m. gilfanov r. sunyaev e. chtirazov (eds.) universe: the most luminous celestial objects and their use for cosmology.
Table of contents
- Intracluster medium
- Fan et al., Evolution of the Ionizing Background
- Kundrecensioner
- Boosting jet power in black hole spacetimes
The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to megakelvins , emitting strong X-ray radiation. The ICM is composed primarily of ordinary baryons , mainly ionised hydrogen and helium. The amount of heavier elements relative to hydrogen, known as metallicity in astronomy, is roughly a third of the value in the sun. Studying the chemical composition of typical ICMs [ further explanation needed ] at varying redshift , which corresponds to looking at different epochs of the evolution of the Universe, can provide a history record of element production.
The mean free path of the particles is roughly 10 16 m, or about one lightyear. The density of the ICM rises towards the centre of the cluster with a relatively strong peak. Once the density of the plasma reaches a critical value, enough interactions between the ions ensures cooling via X-ray radiation.
As the ICM is at such high temperatures, it emits X-ray radiation, mainly by the bremsstrahlung process and X-ray emission lines from the heavy elements. Measurements of the temperature and density profiles in galaxy clusters allow for a determination of the mass distribution profile of the ICM through hydrostatic equilibrium modeling. The mass distributions determined from these methods reveal masses that far exceed the luminous mass seen and are thus a strong indication of dark matter in galaxy clusters.
Inverse Compton Scattering of low energy photons through interactions with the relativistic electrons in the ICM cause distortions in the spectrum of the cosmic microwave background radiation CMB , known as the Sunyaev—Zel'dovich effect. These temperature distortions in the CMB can be used by telescopes such as the South Pole Telescope to detect dense clusters of galaxies at high redshifts [7].
Plasma in regions of the cluster, with a cooling time shorter than the age of the system, should be cooling due to strong X-ray radiation where emission is proportional to the density squared. Since the density of the ICM is highest towards the center of the cluster, the radiative cooling time drops a significant amount. This inflow would result in regions of cold gas and thus regions of new star formation.
We find that, remarkably, both the spin and the translational motion of the black hole contribute to this electromagnetic luminosity. In what follows, we conduct numerical experiments to detail how each component scales.
Intracluster medium
To fix ideas, we envision a magnetic field anchored in the disk with its associated magnetic dipole aligned, in the binary case, with the orbital angular momentum chosen along the direction. In the single black hole case, we consider propagation velocities orthogonal to. Our resulting initial configuration thus has the magnetic field perpendicular to the velocity of the black holes and the electric field is set to zero.
Because the electromagnetic field is affected by the spacetime curvature, it is dynamically distorted from its initial configuration and generates a transient burst of electromagnetic waves as it settles into a physically relevant and dynamical configuration. This value is chosen to be consistent with possible astrophysical magnetic field strengths 20 , Thus, although the plasma is profoundly affected by the black holes, it has a negligible influence on the black holes.
We provide proportionality factors for other quantities of interest. The result captured in Eq. In addition, eccentric orbits and spin—orbit interactions driving orbital plane precession can induce important variabilities in the luminosity, which can also aid in the detection of these systems.
We first consider the radiation from a single black hole immersed in a magnetic field anchored by currents in a distant circumbinary disk. Such a black hole can be spinning and can also have some constant velocity with respect to the initially uniform magnetic field. We calculate the resulting electromagnetic collimated flux energy and its dependence on both velocity and black hole spin. A collimated emission is clearly induced along the asymptotic magnetic field direction. An even stronger emission is obtained for the spinning black hole, as expected.
The color scale is the same for both frames and the black hole is boosted to the right. The stronger emission in the vertically collimated region of the right frame is apparent. We present our results with respect to the velocity, v the measured coordinate velocity of the black hole , and plot in Fig. Collimated luminosity as a function of the boost velocity of the black hole for two different spin magnitudes. The constant L 2 is the same for both fits, supporting the argument that the luminosity does contain two different components, one for spin and one for boost velocity.
Several key observations are evident from the figure. That there is radiation in the spinning case is expected, as the spinning black hole interacts with a surrounding plasma and radiates by the BZ mechanism 2. In the former case, this flux arises solely from the ability of the system to tap translational kinetic energy from the black hole, whereas the latter results from the extraction of both translational and rotational kinetic energies.
The nonspinning black hole demonstrates an electromagnetic luminosity that increases with v 2 see Fig. This behavior is consistent with treating the black hole within the membrane paradigm of ref. A spinless black hole moving with speed v through an ambient magnetic field behaves as a conductor and acquires an induced charge proportional to the speed 8 , 9 , 25 , In this regard, the charge separation on the surface of the black hole is completely analogous to the classical Hall effect. This quadratic dependence on speed is apparent in the figure.
Fan et al., Evolution of the Ionizing Background
These results are also consistent with the work of Drell et al. With the cautionary note that measuring this angle is ambiguous in the curved spacetime around the black hole, such a relation is indeed manifested in our results. Notice that the difference between the obtained luminosities in the spinning and spinless cases remains fairly constant for the different values of v. The roughly constant offset between spinning and nonspinning cases supports the decomposition given in Eq.
In order to be more definite, we introduce two fitting constants, L 1 and L 2 , such that the general luminosity dependence of Eq. This phenomenology strongly suggests that the Poynting flux can tap both rotational and translational kinetic energies from the black hole and that faster and more rapidly spinning black holes have a stronger associated power output. We turn our attention now to orbiting binary black holes. In particular, we concentrate on three cases: Before discussing the results from these binaries, we first describe our analysis that enables a quantitative comparison between the evolutions.
First, we evaluate the luminosities as functions of gravitational wave frequency, as this is an observable and allows for a direct comparison of the different cases. Second, we compute three different luminosities for each case: These different luminosities are displayed for the three binary configurations in Fig.
Upper Left The collimated luminosity associated with the jets. Upper Right The isotropic luminosity representing electromagnetic flux not associated with the jets. Lower The gravitational wave output. Their qualitative behavior is illustrated in Fig. Extensive numerical simulations see, for instance, ref. The orbiting stage leaves its imprint as twisted tubes.
Kundrecensioner
The binary with individual spins aligned reaches a higher orbital velocity before merger than the previous two cases; thus, the expected maximum power should be higher. As evident from the figure, at low frequencies where the orbital dynamics are the same in all cases, both spinning cases have a higher output than the nonspinning one and the difference is provided by the spin contribution to the jet emission.
Furthermore, both spinning cases have equal collimated power output because the spin contribution to the luminosity depends only on the spin magnitude. We can further examine the basic relation given in Eq. The obtained values for the frequencies considered are as follows: Notice that the remains fairly constant through these frequencies, which is evident also in Fig.
That this value is essentially the same for all cases further supports the basic relation in Eq. Using our measured orbital velocities, Eq. Finally, notice that, at even higher frequencies, Fig. The orbiting behavior leaves a clear imprint in the resulting flux of energy and, as the merger proceeds, emission along all directions is evident.
At late times, the system settles to a single jet as dictated by the BZ mechanism. In all cases, a significant noncollimated emission is induced during the merger phase evident in the upper-right plot of Fig. Clearly, the simple-minded picture of a jet produced by the superposition of the orbital and spinning effect cannot fully capture the complete behavior at the merger epoch, although it serves to understand the main qualitative features and provides a means to estimate the power of the electromagnetic emission.
We have studied the impact of black hole motion through a plasma and indicated how the interaction can induce powerful electromagnetic emissions even for nonspinning black holes.
Boosting jet power in black hole spacetimes
Despite having examined a very small subset of the binary black hole parameter space, the results presented both here and in refs. Moreover, as the plasma generally has a negligible effect on the dynamics of the black holes, one needs only to know the dynamics of the black holes, say by numerical solution or other approximate methods, in order to estimate the expected electromagnetic luminosity.
Naturally, spinning black holes produce stronger jets, and these jets, as shown earlier 9 , will be aligned with the asymptotic magnetic field direction. In particular, the luminosity tied to the motion can become significantly higher than that tied to the spin. In addition, eccentric orbits and spin—orbit interactions driving orbital plane precessions can induce important variabilities that can aid in the detection of these systems. Furthermore, a significant pulse of nearly isotropic radiation is emitted during merger, thereby allowing observations of the system along directions not aligned with the jet.
Consequently, binary black hole interactions with surrounding plasmas can yield powerful electromagnetic outputs and allow for observing these systems through both gravitational and electromagnetic radiation. Gravitational waves from these systems corresponding to the last year before the merger could be observed to large distances with the Laser Interferometer Space Antenna up to redshifts of 5—10, ref. As we have indicated here, both scenarios can have strong associated electromagnetic emissions.
The combined gravitational and electromagnetic systems that we consider consist of black hole spacetimes in which the black holes can be regarded as immersed in an external magnetic field. Such fields, as mentioned earlier, will be anchored to a disk. We consider this disk to be outside our computational domain but its influence is realized through the imposition of suitable boundary conditions on the incoming electric and magnetic field modes on the boundaries of our domain 8 , 9. These conditions essentially correspond to setting to zero the electric field while constant values for the magnetic field given by.
With regard to the magnetosphere around the black holes, we assume that the energy density of the magnetic field dominates over its tenuous density such that the inertia of the plasma can be neglected. The magnetosphere is therefore treated within the force-free approximation 2 , We note that the contribution of the energy density of the plasma to the dynamics of the spacetime is negligible and we can ignore its back reaction on the spacetime. We use the Baumgarte—Shapiro—Shibata—Nakamura formulation 34 , 35 of the Einstein equations and the force-free equations as described in refs. We discretize the equations using finite difference techniques on a regular Cartesian grid and use adaptive mesh refinement AMR to ensure that sufficient resolution is available where required in an efficient manner.
We use the Hybrid Adaptive computational infrastructure, which provides distributed, Berger—Oliger style AMR 37 , 38 with full subcycling in time, together with an improved treatment of artificial boundaries Refinement regions are determined using truncation error estimation provided by a shadow hierarchy 40 , which adapts dynamically to ensure the estimated error is bounded by a prespecified tolerance. Typically our adopted values result in a grid hierarchy yielding a resolution such that 40 grid points in each direction cover each black hole.
We use a fourth-order accurate spatial discretization and a third-order accurate in time Runge—Kutta integration scheme In tests performed here for the coupled system and in our previous works for the force-free Maxwell equations , the code demonstrates convergence while maintaining small constraint residuals for orbiting black holes. Furthermore, we obtain for orbiting black hole evolutions agreement with runs from other codes for the same initial data.
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These scalars are computed by contracting the Maxwell and the Weyl tensors, respectively, with a suitably defined null tetrad as discussed in ref. However, as the system studied here has a ubiquitous magnetic field, special care must be taken to compute the energy flux.
The luminosities in electromagnetic and gravitational waves are given by the integrals of the fluxes [6] [7]. The authors thank J. Thompson, as well as our long time collaborators Matthew Anderson, Miguel Megevand, and Oscar Reula for useful discussions and comments.
The orthogonal case is only half as powerful as the aligned case. We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address. Skip to main content. Hirschmann , Steven L. Liebling , Patrick M.